Novel chemical looping oxidation of biomass-derived carbohydrates to super-high-yield formic acid using heteropolyacids as oxygen carrier

Abstract

In recent years, formic acid (FA) shows great potential in fuel cells and hydrogen storage. For FA production from renewable resources, catalytic aerobic oxidation of biomass to FA has been reported. However, the yield of by-product CO2 is fairly high, limiting the FA selectivity and causing greenhouse gas emission. Herein, a novel chemical looping oxidation of biomass-derived carbohydrates to FA was first proposed. A heteropolyacid H8PV5Mo7O40 (HPA-5) was selected as an excellent oxygen carrier. Oxidation of carbohydrates to FA and re-oxidation of reduced HPA-5 (HPA-5red) by O2 were separately performed, by which the FA yield from glucose achieved 95.4%. The extremely high yield of FA is attributed to two reasons. On one hand, the chemical looping process avoids over-oxidation caused by the hydroxyl radicals formed in the re-oxidation of HPA-5red. On the other hand, abundant HPA-5 promotes the glucose oxidation to FA, suppressing the acid-catalyzed decomposition and the oxidation toward saccharic acids. Besides, HPA-5red can be completely re-oxidized by O2 and the chemical looping oxidation has stably performed well in five chemical cycles. This oxidation strategy also shows outstanding FA yields from xylan, cellulose and raw biomass.